Numerous studies have shown that the air traffic services (ATS) community is running out of VHF (Very High Frequency) communication channels. This is especially true in the Northeast corridor of the United States and in Western Europe. The European solution has been to sub-divide the 25 kHz VHF voice channels into 8.33 kHz channels (that is, channels having an 8.33 kHz separation). The solution chosen by the United States is to multiplex up to four digital channels onto a single 25 kHz VHF channel, which can be used for either digital voice or data.
In oceanic and other remote parts of the world, the available voice communications for ATS is over HF (High Frequency) SSB (Single Side Band), which suffers from noise and variable propagation problems. Some of these problems have been mitigated for the air traffic controller by using radio operators to serve as a buffer between the controller and the radio. Airplane pilots flying in these regions, however, must continue to bear the burden of communicating in this bad environment. In other than polar regions, satellite voice has been developed, although it has not been put into regular use.
Under conventional practices, a pilot and controller normally communicate via VHF analog voice radio. A VHF channel is assigned to a particular air traffic control (ATC) sector. When an aircraft approaches that sector, the pilot is directed to tune the aircraft radio to the channel assigned to the sector. As a result, pilots are often required to change radio frequencies numerous times during a flight. Furthermore, additional separate frequencies are usually required for ground and approach control at each airport capable of handling commercial aviation traffic.
When a pilot or controller needs to transmit a message, (s)he first listens to the channel and waits for a quiet period in the traffic on that channel. For most channels, the transmissions are short and traffic is light enough that the wait is relatively short. In some highly-congested channels, however, the wait can be 30 seconds or more. During this time the pilot or controller must hold the message in a mental queue and give partial attention to the voice traffic. Judgment and experience are applied to determine whether a silence is the appropriate break for the sender's own message or if it is only a turn-around in an ongoing dialog. At the appropriate time, the pilot or controller presses the push-to-talk (PTT) switch and states the message, as rapidly and succinctly as possible, to minimize channel occupancy. Procedure dictates that the call sign of the intended receiver, followed by the call sign of the originator, be included at the beginning of the message, e.g., “Seattle Center, United 234 . . . ” S(he) then listens to the channel for indication that the message was received without corruption. Failure to hear a valid acknowledgement might mean that two transmitters were activated at the same time and both failed to communicate or it may mean that the receiving person failed to hear the message. In either case the message must be retransmitted.
In addition to the air traffic control channel, pilots need to communicate with other ground entities. U.S. carriers are required by U.S. 14 CFR 121.99 to have communication capability with their dispatch center. Other carriers have similar requirements, either by law or by standard operating procedure. General aviation flights also have the need to contact ground service entities for weather and other operational messages. As a result, most aircraft are equipped with a minimum of two VHF communication radios and many have more radios to support their routes and operational needs.
The VHF radio for airline operational communications (AOC) has low usage for any one aircraft but traffic for other aircraft is present. In order to allow the flight crew to monitor these additional radios without the additional aural workload, a form of tone annunciation called SELCAL (selective calling) is provided. A call on a channel guarded by SELCAL requires that the originator enter the aircraft SELCAL address before making the initial call. A light and/or a tone annunciates an incoming call. The flight crew, when they see or hear the annunciation, activate the receiver audio and respond.